Modem data processing systems such as computers and other equipment require large capacity mass data storage devices. A common storage device is the rotating magnetic disk drive. A typical rotating magnetic disk drive contains one or more disks mounted on a common spindle. Data is recorded on magnetically encoded "tracks" located on the flat surfaces of the disks. Typically, both surfaces of a disk are used for recording data, although in some designs a single surface is used. A movable "actuator" positions a read/write transducer head adjacent one of the tracks to read or write data on the track.
Such magnetic disk storage devices have been extensively used to store computer programs, text files, databases, etc. Typically, conventional computer data of this sort is contained in small chunks, or records. These records are "small" in the sense that they are small in relation to the total volume of storage available on the disk drive. A disk drive will therefore contain a large number of such records, any one of which might be needed by the computer system at a moment's notice. A disk drive must be able to rapidly respond to a request by the computer system for any record stored on the disk.
The type of data stored on disk drives, and the need to access the data rapidly no matter where it is stored, have guided the design of conventional disk drive storage devices. In order to enable the disk drive to rapidly access data anywhere on the surface of the disk, the data tracks are arranged as a series of concentric rings. When a new piece of data is accessed, the actuator must move to the appropriate track (ring), which is known as a "seek" operation. The time required to move the actuator to a new track is the "seek" time. In order to reduce seek time, a powerful electromagnetic motor moves the actuator at great speed from one track to another, across the surface of the disk. But that is not all. Once the actuator has been positioned at the appropriate track, it must wait for the disk to rotate until the desired data is directly adjacent to the transducer head. Sometimes the data will be adjacent to the transducer almost immediately, but at other times the drive must wait for the disk to complete nearly a full revolution before the data is available. The time required to wait until the data is directly adjacent the transducer is known as the latency time, and on the average it will be the time required to complete one-half a disk revolution. To reduce latency, disk drives have been designed to spin the disks at ever faster speeds.
Recently computers and related devices have begun to store different types of data, collectively known as "multimedia" data, on their mass storage devices. Multimedia data is a digitally encoded form of images, such as digitally encoded photographs, motion pictures, animated presentations, music, and other visual and audio images.
Multimedia data imposes a different set of storage requirements from those of traditional computer data. On the one hand, multimedia records are massive, and so the demands for storage devices of much larger capacity are increased. On the other hand, multimedia does not require rapid access to any random part of the record, nor does it require the very low error rams of conventional computer data.
While it is possible to store multimedia data on conventional rotating magnetic disk storage devices, the designs of these devices have been optimized for conventional computer data. There is a need for magnetic disk storage devices which are designed for efficient storage of multimedia data.
A particular application for mass storage of multimedia data is in a "video-on-demand" system. "Video-on-demand" is a system that will provide a user with a video selection from a large library at any time of day. Several such systems have been proposed, but functioning video-on-demand systems are not yet commercially available. To date, there has been no consensus as to how such systems should be constructed, in part due to the lack of storage devices designed for efficient storage of multimedia data.